The ITU-T standard defining interchange circuits between DTE and DCE. V.24 is the ITU-T equivalent of EIA standard EIA-232C, though V.24 only specifies the meaning of the signals, not the connector or the voltages used.
V.24 recommends 12 modem carrier frequencies that will not interfere with Dual Tone Multi-Frequency or other telephone control tones. These are:
GROUP A = 920 Hz, 1000 Hz, 1080 Hz, 1160 Hz GROUP B = 1320 Hz, 1400 Hz, 1480 Hz, 1560 Hz Group C = 1720 Hz, 1800 Hz, 1880 Hz, 1960 Hz